It is known in the product sector relating to the packaging of bulk material that there exists the need to introduce said material inside bags, which must be filled with a given and precisely defined quantity of material and then sealed. It is also known that, for this purpose, automatic bag filling machines have been developed, an example of said machines consisting of so-called form, fill and seal machines, which are able to perform at high speed the cycle of forming the bag, filling the bag and final sealing of the bag mouth. These machines comprise in particular a station for filling the bag with the product contained inside a hopper arranged in a position substantially coaxial with the bag itself and higher than the level of the bag mouth, said quantity of filling product being able to be determined substantially using three main methods referred to as follows: a net-weight method (i.e., involving weighing the product before it is introduced into the bag); a gross-weight method (i.e., involving weighing the product together with the filling bag; and a volumetric method (i.e., where a predefined volume of product is prepared independently of the measurement of its overall weight).
Machines of this type, which are known for example from EP 0,595,778 in the name of the same present Applicant, comprise a bag filling step which is performed by allowing the product to fall by means of gravity inside the bag, which is filled with given quantities determined using one of the aforementioned metering methods. FIGS. 1 and 2 show a form fill seal (FFS) machine with a forming station F, a filling station R with fixed grippers for gripping the bag engaged with the product feed mouth, which station is provided with rotating valves 210 able to open and close; a station S for sealing the bag mouth and a conveyor 113 for conveying out of the machine the filled and sealed bag; FIG. 2 also shows in schematic form the working sequence performed by means for conveying the bag from one station to another with opening of the mouth, closing of the mouth after filling, and sealing of the mouth with release of the grippers.
Although fulfilling its intended function, this bag filling operation, which is performed in a single step, involves a number of problems due to the fact that the falling movement of the product generates a large quantity of dust, which tends to spread outwards, being dispersed in the environment or resulting in the need to use costly suction devices. In addition, with single-step filling, a large amount of air is also introduced into the bag and must be expelled to allow correct filling with the correct metered quantity of material. In addition to this, products such as cement products, finely ground calcium carbonates, powder milk, starches and the like have the characteristic feature that they increase in volume by more than 50% in terms of their own particular weight unit, owing to the air, which is trapped between the molecules of material. These products inside the bags, if not suitably treated, have a high instability and consequently require specific treatment for reduction of their volume and palletization so that they are ready for commercial distribution. In addition, it is also mentioned that a suitable compaction of the volume of product introduced into the bag results in a reduction of the material needed for packaging, thereby reducing the final cost and the impact on the environment.
In order to perform the packaging of these materials, auxiliary mechanisms such as vibrators and porous suction probes are used, these being designed to remove the air from the material already introduced into the bag and being installed on automatic machines, such as the abovementioned FFS machines for forming, filling and sealing the bag.
In addition to the above, a further problem is mentioned, i.e., that due to the fact that the apparatus for metering the product to be filled inside the bags are always affected by a certain degree of uncertainty with regard to the measurement operation. In fact, in the case of net-weight weighing, the quantity of product actually present inside the bag is approximate, with consequent weighing errors.